A turbofan is a widely used type of a gas turbine engine. The distinctive feature of the turbofan engine is an axial flow fan. The discharge air from the fan contributes substantially to the overall thrust of the engine. The amount of thrust generated by an engine with a given core is a function of the size of the fan. Longer fan blades for the given core engine accelerate more air through the fan and correspond to a higher thrust engine.
The fan, disposed in the forward portion of the engine, is enclosed in a duct and equipped with rotating blades and stationary vanes. The fan blades operate in a very hostile environment and therefore must be designed to withstand large loads, impact, and vibrations. Each blade comprises an airfoil portion and a dovetail root portion secured in a fan disk. As the fan rotates at very high speeds, the blades are subjected to extreme centrifugal loads in a radial direction. The centrifugal forces produce large tensile stresses within each blade. The reaction force from the disk which retains the blade manifests itself as a compressive force applied to the dovetail root portion. In order to overcome such an extreme stress load and avoid frequent repairs, each fan blade must meet stringent design criteria. In addition, the blades must be light weight to optimize the efficiency of the engine.
Conventional fan blades are fabricated from titanium and tend to be solid for two reasons. First, the compressive force acting on the fan blade dovetail root may collapse the blade, if it is not made solid. Second, until recently, hollow fan blade fabrication, while desirable from a weight standpoint, has been cost prohibitive. The airfoil portion is fabricated to be very thin in order to minimize the weight of the blades. However, high performance modern engines, producing high levels of thrust, necessarily command a longer blade. A problem arises since very thin, long blades produce vibratory flutter because they cannot support themselves sufficiently. To reduce such flutter problems, mid-span shrouds or braces are placed between the blades in the mid-section thereof. The drawback associated with the mid-span shrouds is that the shrouds form a continuous ring in the mid-section of the fan blade, impending the airflow and thereby reducing efficiency in performance.
One method to eliminate the inefficiency in performance is to remove the mid-span shrouds. The common approach to compensate for the lack of mid-span shrouds and maintain flutter free blades, is to increase the chord length and the thickness of the blade. However, the increase in length and thickness of the blade creates a prohibitive weight penalty in each blade, and subsequently, in the entire engine. Since it is critical for the engine to be as light weight as possible, the airfoil portion of the fan blade is currently hollow. However, another problem arises with such blades. In general, stress concentration is a result of geometrical discontinuity. In hollow fan blades, stress concentrations occur at the transition point between the hollow part of the airfoil and the solid root thereof. When the stress concentration area coincides with an area experiencing high centrifugal loading, the integrity of the fan blade can be jeopardized. In a fan blade having a hollow airfoil portion, the transition area between the hollow airfoil and the solid dovetail root does occur in the area subjected to high centrifugal loading and presents a threat to the integrity of the fan blade. Thus, there is a great need for light weight, aerodynamically efficient and reliable fan blades.